1. Field of the Invention
This invention relates to a method and apparatus for humidifying air using the water vapor from moisture-laden flue gas. More particularly, this invention relates to a method and apparatus for humidifying air using a permselective water transport membrane in which water vapor from the moisture-laden flue gas is condensed and the condensed water is transported through the membrane for evaporation into an air stream.
2. Description of Related Art
Methods and apparatuses for the selective removal of one or more components from a gaseous mixture are well-known. U.S. Pat. No. 5,753,009 teaches a method and apparatus for selective removal of one or more components from a multi-component gas/vapor mixture by membrane fractionation. The membrane fractionation gas removal system comprises a feed chamber containing gas/vapor mixture, at least one porous membrane having a first side which contacts the gas/vapor mixture in the feed chamber, at least one non-porous membrane having one side which contacts the second side of the porous membrane, which non-porous membrane has a permeability selective to one or more components of the gas/vapor mixture, an exit chamber connected to the second side of the non-porous membrane such that the component exiting the non-porous membrane enters the exit chamber, and an evacuation member connected to the exit chamber for evacuating one or more components from within the exit chamber. U.S. Pat. No. 4,875,908 teaches a process for selectively separating water vapor from a multi-component gaseous mixture in which the multi-component gaseous mixture comprising the water vapor is passed along and in contact with a membrane which is selectively permeable to water vapor. The use of membranes for selective removal of one or more components of a gaseous mixture is also taught by U.S. Pat. No. 4,583,996 (inorganic porous membrane), U.S. Pat. No. 3,980,605 (fibrous semi-permeable membrane), and U.S. Pat. No. 3,735,559 (sulfonated polyxylylene oxide membranes).
Methods and apparatuses for selective removal of water vapor from a gaseous mixture and condensing the separated water vapor to recover its latent heat of vaporization are also known. U.S. Pat. No. 5,236,474 teaches a process for removing and recovering a condensable vapor from a gas stream by a membrane contactor in which a gas stream containing a condensable vapor is circulated on one side of hollow fiber membranes while cool extraction fluid is circulated on the other side under a total pressure differential. As a result, the condensable vapor in the gas stream is condensed in the gas stream and the condensed vapor, i.e. liquid, permeates the membrane and becomes entrained in the cool extraction fluid. U.S. Pat. No. 6,517,607 B2 teaches a method for removing a condensable component from a process stream in which a first side of a permselective membrane is contacted with a process stream in which is disposed the condensable component. The condensable component is passed through a permselective membrane to a second side of the permselective membrane, forming a condensable permeate. The condensable permeate is then contacted with a liquid stream having a liquid form of the condensable permeate, forming a condensed permeate, which may then be returned to the process which generated process stream.
In colder climates, air within buildings tends to be dry as a result of which humidification is frequently employed to add more moisture to the air to keep people within the building comfortable and to prevent damage to dry-sensitive objects within the building. Conventional means for humidifying air include adding water by spray, atomizers, or distributors. Typically, in a house, a whole house humidifier installed with the furnace is the best way to humidify all the air in the house. A typical residential humidifier requires a water supply to distribute water along a filter such that hot and dry air coming through the filter will evaporate some of the water so that the air can be humidified. To ensure sufficient wetting of the filter and minimize deposition of dissolved minerals on the filter, a portion of the water supplied to the filter is wasted and goes down to the drain.
Typically, 12 gallons per day of water needs to be added to the air for a house with 3000 ft2. Considering the wasted water to the drain, a consumption of 20 gallons per day of water is typical. However, in addition to conventional water supplies, water is also present in the residential gas furnace flue gas. For example, for a typical 110,000 BTU/hour residential gas furnace flue gas, approximately 30.75 gallons of water per day could be available for this purpose. Extraction of 40% of this water could provide about 12.3 gallons per day, which is enough to meet the typical residential air humidification demand. However, flue gas typically contains contaminants which must be prevented from mixing with the water in order to avoid contamination of the building air supply.